The transition from mitosis to meiosis in the yeast Saccharomyces cerevisiae is initiated by placing growing cells under pseudo-starvation conditions. The ability of diploid cells to undergo meiosis is controlled primarily by the mating type locus. We are trying to understand the structure of the mating type locus and the manner by which one mating type allele can be replaced by another. This process involves some process of transposition of genetic information from usually silent locations to the mating type locus. We are especially studying a number of mutations that inhibit mating type switching. We are also studying changes in gene expression that accompany the shift from growth to sporulation. These studies include an analysis of changes in protein synthesis and the action of several mutations that control ribosomal protein synthesis.